French patent Application No. 583,590 or U.S. Pat. No. 4,689,713 to Hourtane et al relates to a device for protecting a polyphase electric power line against high transient overvoltages of electromagnetic origin, comprising at least two stages in series, the first stage being provided with a spark-gap and an inductor for decoupling with respect to the following stage and being followed by a stage having a bridge limiter including two oppositely-biased power diodes each connected in series with a high-value capacitor, a time-delay contactor being placed at the terminals of each power-diode/capacitor assembly so as to permit charging of the capacitors through a resistor and auxiliary diodes placed between on the one hand the capacitor and on the other hand the connection between the power diode and the capacitor, then to permit cutoff of the bridge limiter once all the capacitors have been charged.
Although the device disclosed in the patent Application cited in the foregoing does in fact ensure effective protection and limits current surges at the time of startup, the device nevertheless makes it necessary in the case of a polyphase network to employ two high-value capacitors on each phase as well as a resistor and two auxiliary diodes. This device consequently suffers from a certain degree of complexity as well as bulk.